[sdiy] Tr-808 issue

el macaco elmacaco at hotmail.com
Thu Sep 13 23:36:17 CEST 2018


Thanks everyone for the suggestions!

Ok, so the 808 sometimes works and sometimes doesn’t.  Lately it just doesn’t.  

I’ve replaced everything on the psi board, diodes caps rectifiers regulators resistors.

And if I remember right, everything in the protection circuit sk30a’s and 945 transistors, I don’t have any 733’s so they may still be an issue, but it appears the protection circuit is working because when I measure the voltages when it doesn’t work they are uneven.

I will try and get my scope fixed in town or have someone bring a cheap small one next visit from civilization, it’ll be a while tho.

I have continuity tested traces around the protection circuit but may as well test them all while I wait for parts.

It’s a 117v 808, was not connected to 220vac, just shitty inverter.

Could a faulty transformer provide the uneven power rails? Since everything else is pretty new....

I did do a lot of resoldering of the protection circuit and visual inspection, but will do more  now.

The jungle is a bit humid ( I’m not in lowland jungle, more mountainous region)

But part leads tarnish and I fixed a few other things by reseating chips or spraying contact cleaner on them in sockets.  

No sockets in the 808, but might indicate somewhere to look for those in the know?

Was also thinking of the bypass electrolytic capacitors, might as well replace the 40year old ones eventually, but 25 of them when I’m not sure it’s the issue is holding me back.  

Thanks to all for the directions!  I’ll apply what so far has been said but keep any ideas coming!

El

> On Sep 13, 2018, at 3:53 PM, Kenny Balys <kenny at beatkamp.com> wrote:
> 
> 
> 
> It shouldn't do that, a square AC main will be a source of
> noise but the transforer should be able to handle it.
> 
> Is there no way for you to get a scope? There may be more than one
> issue and others are sure to come up.
> 
> With 808's you could have a bad trace on the motherboard that
> comes and goes and this could make one think that there are bad
> components.
> 
> If you haven't already, I would suggest going through and resoldering
> every point in the trouble area and beyond and making a very close
> visual inspection.
> 
> I had a Jupiter-6 that drove me completely bonkers until I found an
> intermittent bad trace. It only went bad once the machine heated up.
> The problem did not manifest when the machine was cool.
> 
> 
> 
> 
>> On 13.09.18 20:33 , el macaco wrote:
>> Am I on the right track thinking a squarish 120vac could bugger the transformer?
>> 
>>> On Sep 13, 2018, at 3:26 PM, el macaco <elmacaco at hotmail.com> wrote:
>>> 
>>> Yes.  Thanks!! I have a true sine wave inverter now and it’s solid and even goes through line conditioners etc before the 808.  All the other gear works fine on it.
>>> 
>>> The earlier set up was before I had a proper solar set up.
>>> 
>>> 
>>> 
>>>> On Sep 13, 2018, at 2:42 PM, Kenny Balys <kenny at beatkamp.com> wrote:
>>>> 
>>>> 
>>>> Are you certain of the AC power coming in to the 808?
>>>> 
>>>> I would suspect that as much as the power transformer.
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>>> On 13.09.18 19:30 , el macaco wrote:
>>>>> Hello all,
>>>>> 
>>>>> I’m asking for guidance to fix my Tr-808.  I usually fix my gear myself but this has me stumped.
>>>>> 
>>>>> I live in the amazon far from any reliable techs and have mostly serviced my own stuff over the years with all I have learned from this list since 1999.
>>>>> 
>>>>> My TR-808 has an issue where the voltage protection circuit kicks in and doesn’t let it run.  This happens when the power is unstable in the 808 and is by design.
>>>>> 
>>>>> It first happened after running it from a solar power system with a voltage inverter that wasn’t pure sine wave power.
>>>>> 
>>>>> The power supply was giving uneven power, 15.6v and -14v.
>>>>> 
>>>>> Replacing the parts in the power supply, rectifier diodes, capacitors, regulators, resistors helped some but it doesn’t reliably work although the voltages were even 15/-15.
>>>>> 
>>>>> The only power supply part I haven’t replaced is the transformer, which puts out static 26v AC voltage as required, but my oscilloscope is busted so it’s with a DMM so I can’t see if there’s ripple or other problems.
>>>>> 
>>>>> That’s the next part I need to order, the transformer.
>>>>> 
>>>>> Does anyone know how to test the transformer without a scope?
>>>>> 
>>>>> Or is there something else that can cause this protection circuit to engage?
>>>>> 
>>>>> I also replaced capacitors and transistors in the protection circuit in case that was the issue, it helped some initially but it still only works occasionally.
>>>>> 
>>>>> The service manual
>>>>> Mentions something about malfunctioning IC can cause this circuit to engage.
>>>>> 
>>>>> If the transformer doesn’t help next on the list is replacing the electrolytic capacitors that filter the power at each circuit (25 total I think).
>>>>> 
>>>>> Any ideas greatly appreciated.
>>>>> 
>>> 




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